The invention generally relates to integrating an FM transmitter into a cellular telephone and more particularly relates to an FM transmitter that has features that facilitate integration of the transmitter into a cellular telephone.
A modern cellular telephone may have the capability of playing digital music files (MP3 files, for example). Due to the limited capability of the cellular telephone's speaker system, the telephone may contain a low power frequency modulation (FM) transmitter for purposes of communicating the digital music file content over an FM channel to a nearby stereo system. However, a potential challenge with incorporating an FM transmitter into a cellular telephone is that out of band spectral energy that is generated by the FM transmitter may encroach into the receive channels of the telephone, thereby potentially impairing the telephone's ability to receive an incoming signal.
As a more specific example, for the Global System for Mobile communications (GSM) standard, the RF signal that is received by the cellular telephone may have a relatively small magnitude, such as about −108 dBm. Any spectral energy (such as out of band spectral energy that is generated by the FM transmitter, for example) that appears in the GSM receive channel must be smaller than the noise floor of the receive channel, which may be approximately −117 dBm. Therefore, stringent requirements may be placed on the out of band spectral energy that is transmitted by the FM transmitter.
Thus, there is a continuing need for better ways to integrate an FM transmitter and a cellular telephone.